The invention relates generally to radiographic detectors for diagnostic imaging, and more particularly to enhancing the flux rate in direct conversion detectors for high flux rate imaging with photon counting and energy discrimination, such as in computed tomography (CT) applications.
Radiographic imaging systems, such as X-ray and computed tomography (CT) have been employed for observing, in real time, interior aspects of an object. Typically, the imaging systems include an X-ray source that is configured to emit X-rays toward an object of interest, such as a patient or a piece of luggage. A detecting device, such as an array of radiation detectors, is positioned on the other side of the object and is configured to detect the X-rays transmitted through the object.
Conventional CT and other radiographic imaging systems utilize detectors that convert radiographic energy into current signals that are integrated over a time period, then measured and ultimately digitized. A drawback of such detectors however is their inability to provide data or feedback as to the number and/or energy of photons detected. Also, energy discriminating, direct conversion detectors capable of not only X-ray counting, but also providing a measurement of the energy level of each X-ray detected have been employed in CT systems. However, a drawback of these direct conversion semiconductor detectors is their inability to count at the X-ray photon flux rates typically encountered with conventional CT systems. Further, the very high X-ray photon flux rate has been known to cause pile-up and polarization that ultimately leads to detector saturation. “Pile-up” is a phenomenon that occurs when a source flux at the detector is so bright that there is a non-negligible possibility that two or more X-ray photons deposit charge packets in a single pixel (“photon pile-up”), or in neighboring pixels (“pattern pile-up”), during one read-out cycle (i.e., one frame). In such a case these events are recognized as one single event having the sum of their energies. If this happens sufficiently often, this will result in a hardening of the spectrum as piled-up soft events are shifted in the spectrum to higher energies. In addition, pile-up leads to a more or less pronounced depression of counts in a central part of a bright source, resulting in flux loss. Pile-up also affects light curves, suppressing high count rates. In other words, these detectors typically saturate at relatively low X-ray flux level thresholds. Above these thresholds, the detector response is not predictable or has degraded dose utilization. That is, once a pixel is saturated (corresponding to a bright spot in the generated signal), additional radiation will not produce useful detail in the image.
Further, as will be appreciated, detector saturation leads to loss of imaging information and consequently results in noise and artifacts in X-ray projection and CT images. Photon counting direct conversion detectors are known to suffer from decreased detector quantum efficiency (DQE) at high count rates mainly due to detector pile-up. In particular, photon counting direct conversion detectors, show pile up due to the intrinsic charge collection time (i.e., dead time) associated with each X-ray photon event. As indicated above, saturation ultimately is often due to pulse pile-up, particularly when the X-ray photon absorption rate for each pixel is on the order of the inverse of this charge collection time. The reciprocal of the charge collection time is called a maximum periodic rate (MPR). When the true mean X-ray count rate incident on the detector is equal to the maximum periodic rate, the DQE is one half and the output count rate recorded is only one half the maximum periodic rate. Reduced DQE results in reduced image quality, i.e., a noisy image. In addition, hysteresis and other non-linear effects occur at flux levels near detector saturation as well as flux levels over detector saturation and lead to image artifacts.
In addition, the relationship between the true signal and the measured signal becomes non-linear, dropping off as the count rate is increased. This pile-up effect, if stable, may then be calibrated and corrected, thereby increasing the effective count rate capability of the detector, albeit with a penalty of higher noise. However, if the count rate is increased to a point where the relationship between the true signal and the measured signal becomes non-monotonic, correction of this non-monotonic relationship may no longer be practical. In this case the detector is over-ranged, and the count rate at this point becomes the maximum achievable count rate.
Previously conceived solutions to enable photon counting at high X-ray flux rates include using bowtie shaped filters to pre-condition the flux rate at the detector, compensating for the patient shape. Also it has been proposed to subdivide the pixel into multiple sub-pixels, each sub-pixel connected to its own preamplifier. By reducing the area of the direct conversion sub-pixel the flux rate capability may be increased as fewer photons are collected in the smaller area. However, the signal-to-noise ratio of the resulting signal may be reduced, and the level of cross-talk will be disadvantageously significant due to the increased perimeter between sub-pixels.
There is therefore a need for an energy discriminating detector that does not saturate at the X-ray photon flux rates typically found in conventional radiographic systems. In particular, there is a significant need for a design that advantageously combines information from a direct conversion photon counting detector in an optimal way, taking into account associated noise in order to extend the flux rate capability. It would be desirable to improve the flux rate in direct conversion detectors that will allow photon counting in medical and industrial applications that are heretofore unmanageable because either the flux rate or the dynamic range requirements are too high. Additionally, there is a particular need for correction algorithms for known deleterious effects, such as pile-up and pixel over-range.